Apparatus for passing charged particles through a field free region and neutralizingsaid particles during transit



M. R. CURRIE ET AL 3,387,176

June 4, 1968 APPARATUS FOR PASSING CHARGED PARTICLES THROUGH A FIELD FREE REGION AND NEUTRALIZING SAID PARTICLES DURING TRANSIT Original Filed Oct. l2, 1960 2 Sheets-Sheet l JWMMNIIII DNMN.

Tw @if vJune 4, 1968 M. R. 'CURRIE ET AI. 3,387,176

APPARATUS FOR PASSING CHARGED PARTIcLEs THROUGH A FIELD FREE REGION AND NEUTRALIZING sAID PARTIcLEs DURING TRANSIT Original Filed Oct. l2. 1960 2 Sheets-Sheet United States Patent O 3,387,176 APPARATUS FDR PASSING CHARGED PARTICLES THRUUGH A FIELD FREE REGION AND NEU- TRALIZIN G SAID PARTICLES DURNG TRANSIT Malcolm R. Currie, Pacific Palisades, and George R.

Brewer, Malibu, Calif., assignors to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Continuation of application Ser. No. 62,292, Oct. 12, 1960. This application Ian. 5, 1967, Ser. No. 607,394 16 Claims. (Cl. 315111) ABSTRACT F THE DISCLOSURE A method and apparatus for neutralizing the charge of a beam of charged particles wherein the beam is passed through a field-free region which is defined by potential barriers at the upstream and downstream ends of said region, neutralizing particles being introduced into said beam in said region.

This is a continuation of application Ser. No. 62,292, filed Oct. 12, 1960, now abandoned.

This invention relates to a system for effecting neutralization of a beam of charged particles and particularly to a self-regulating neutralizing chamber useful in an ion engine for effectively neutralizingl a beam of positively charged ions.

One of the most critical problems in the development of ion `engines that emit a beam of heavy ions to develop a thrust to propel a vehicle or space craft is to provide effective and reliable neutralization of the beam of positively charged ions. A first requirement of a neutralization system is to provide a net charge neutrality for the craft in order to prevent the attraction of particles of opposite sign back to the craft. In order to provide charge neutrality, the total neutralizing or electron charge emitted must be equal to the total ion charge emitted, averaged over a reasonable period of time.

A second requirement of a neutralization system is that the neutralized beam be ejected from the ion engine in such a way as to prevent the creation of local image charges around the exit region of the ion beam. In or- A der to meet this second requirement, a condition must be developed wherein essentially no electric iiux lines emanate from the neutralized ion beam, that is, that the ion and electron charge per unit length in the neutralized beam be substantially zero. Consistent with the rst requirement that the ion and electron charge emitted per unit time be equal, the axial velocity components of the electrons and the ions in the neutralized beam must be equal to meet the second requirement. Because of the large mass ratio between an electron and an ion, an emitted electron will undergo an acceleration of the order of 250,000 times that of a cesium ion, for example, in a given electric lield. In practice, if cesium ions are accelerated by a voltage of 10,000 volts, they attain a velocity which an electron achieves from an acceleration due to a few hundredths of a volt. Thermionically emitted electrons, however, typically have velocities equivalent to about a tenth of a volt potential. It is therefore difficult to obtain electrons from a thermal source which have the same velocity as the ions in a typical and practical ion beam.

It is therefore an object of this invention to provide a system for effectively neutralizing a stream or region of charged particles.

It is a further object of this invention to provide an ion engine that ejects an effectively neutralized beam so as to reliably and continually provide a desired thrust.

3,387,176 Patented June 4, 1968 It is a still further object of this invention to provide a means for neutralizing a beam of ions with electrons injected into the beam so as to move forward with the beam with a substantially constant axial velocity.

Itis another object of this invention to provide a means to match the axial velocities of a beam of ions and of neutralizing electrons moving forward with the beam by accumulating electrons in a manner automatically to control the axial velocity of the electrons.

It is another object of this invention to provide a neutralizing chamber in which a beam of ions if effectively neutralized by an arrangement that accumulates injected electrons and controls the rate of injection of the electrons in such a manner that the electrons move forward in the beam with a substantially constant axial velocity.

Briefly, this invention is a system and method for neutralizing a beam of charged particles, heavy ions for example, by providing a first electrode for accelerating the beam, a second electrode for decelerating the beam and a third electrode for again accelerating the beam so that the mean -beam potential at the second electrode forms a potential well or valley. Neutralizing particles, for example, electrons, having a polarity opposite to that of the charged particles are injected into the potential well from an emitter. The mean beam potential which forms the bottom of the potential well varies in level in response to accumulated neutralizing particles therein so as to allow some neutralizing particles to overcome the potential barrier of the well and escape into the beam with desired values of axial velocities. Thus the system provides a continuous self-regulating action to control the axial velocity of the neutralizing particles escaping into the neutralized beam. The system may also operate so that the eld developed by the mean beam potential controls the number of neutralizing particles injected from the emitter in a regenerative manner so that not only the depth of the well rises and falls, but the rate of injection of neutralizing particles decreases and increases in corresponding manner.

The novel features of this invention, as well as the invention itself, both as to its organization and method of operation, will best be understood from the accompanying description, taken in connection with the accompanying drawings, in which like characters refer to like parts, and in which:

FIG. 1 is a schematic sectional view of the general arrangement of the system for neutralizing charged particles in accordance with this invention;

FIG. 2 is `a potential profile graph plotting negative potential on the axis versus displacement along the axis for explaining the self-regulating neutralizing operation in accordance with the system of FIG. l;

FIG. 3 is a schematic view partially in section showing the specific structure of an ion engi-ne utilizing the neutralization system in accordance with this invention;

FIG. 4 is a graph of the Maxwell-Boltzmann velocity distribution of electrons showing the number of electrons injected into the potential well versus axial velocity for further explaining the self-regulating action of the potential well in accordance with this invention; and

FIG. 5 is a graph of velocity versus number of ions and electrons for explaining the electron axial velocity distribution developed by the system in accordance with this invention.

Referring rst to FIGS. 1 and 2, the general arrangement of the neutralizing system in accordance with this invention will be explained. A source 10- emits a beam of charged particles whichmay be focused and collimated, as indicated by the beam 12 lmoving in the forward direction of arrow 14. The charged particles may, for example, be cesium ions when the system of FIG. 1 represents an ion engine with cesium'as the propellant, or may be any charged particles such as dust or oil droplets, etc. Within the principles in accordance with this invention, the beam 12 may be of a cylindrical for-rn having a center line 13, a hollow beam having a center line 15 or a planar or strip beam having the line 13 as a plane of symmetry. The source 10 and other structure of the system may be appropriately shaped for the form of beam utilized. A first accelerating electrode 18 is disposed forward of the source 10 and adjacent to the beam 14 and in the case of positive particles may be maintained at a potential substantially more negative than the potential of the source 10. Thus, the mean beam potential at the first accelerating electrode 18 has a large negative value VM as shown in the axial potential profile of FIG. 2 where the mean beam potential Versus axial position, is plotted in a negative voltage direction. A decelerating electrode 22, of a neutralizing chamber 21, is disposed adjacent to the beam 12 and forward of the first accelerating electrode 18 to decrease the velocity of the charged particles and to form a potential well for trapping electrons as described below. The decelerating electrode 22 may be disposed along a substantial length of the beam 12 in order to minimize therein electric fields which are parallel to the path of the beam 12. The decelerating electrode 22, maintained at a potential VDI, increases (downward on the curve) the mean beam potential to a more positive potential such as VD, shown in FIG. 2 as a line 26 on the curve.

Disposed forward of the decelerating electrode 22 and adjacent to the beam 12 is a second accelerating electrode 28 which may be maintained at a direct current potential more negative than the decelerating electrode 22 so as to increase the mean beam potential in the negative direction to a potential VA2 to form the downstream wall of a potential Well 30 in the bea-m 12. It is to be noted that the mean beam potential at the decelerating electrode 22 varies over a potential ran-ge between VDl and VD2 as determined by the accumulation of negatively charged particles as discussed below. A course of neutralizing particles 34 is provided adjacent to the decelerating electrode 22 to supply neutralizing particles through an opening 38 into the potential well of the beam 12. The neutralizing particles, which may be negatively charged particles such as electrons, may be effectively injected at the potential Ve. However, the electrons which may be thermally produced by the source 34 vary in energy over a range from VDI to VD2 above and below Ve with a Maxwellian distribution. Thus, when the potential well 30 has the level 26, the majority of electrons have insufficient energy to overcome the potential barrier of a wall 27 thereof. It is to be noted that the potential well 30 has a wall 31 that effectively is an elecron mirror to reflect electrons or other negatively charged neutralizing particles having an axial velocity in the direction of the source 10. The beam 12 along with its neutralizing particles then continues forward in the direction o-f the arrow 1-4 to be used for the desired purpose, for example, as a thrust producing means in the case of an ion engine.

A more specific arrangement of the electron -gun used as a source of neutralizing particles as shown in FIG. 3 includes an ion source 46 having a source of propellant 48 and Kan ionizer 50 for emitting ions therefrom. The source of propellant `48 may be a cesium vaporizing system and the ionizer 50y may be a porous tungsten matrix constructed of tungsten powder fused into a sponge-like body through which cesium gas atoms flow and are ionized by losing an electron on contact with the hot tungsten. The ionizer 50 may be heated so that cesium ions migrating therethrough absorb thermal energy so as to be emitted from lan emitting surface 52 which may be concave to provide a larger emitting area. The emitting surface 52 is centered on a center-line 54 and emits a converging ion stream or beam 56 having in this example a transverse circular shape. The stream of ions is then projected forward along the axis 54 past a suitable focusing or profile shaping electrode 6) which provides an electric eld to aid in converging and collimating the ion beam. As noted in connection with FIG. 1, the sectional view may also represent a planar beam gun or hollow cylindrical bea-m gun structure. The focusing electrode 60 may have internal surfaces `62 and 64 disposed at selected angles from the edge of the ion beam 56 such as is described in Patent No. 2,817,033, Electron Gun by George R. Brewer.

The ionizer 50 may be maintained at a direct current reference potential of zero volts from a source of potential such as the terminal of a battery 68. The focusing electrode {it} may be conveniently maintained at the same potential as the ionizer 50, as shown, although the gun design is not limited to this mode of operation. The ion stream 56, a distance forward of the focusing electrode 6ft, preferably forms a well collimated beam of ions 70 with substantially perfect laminar flow.

Disposed forward of the focusing electrode 6i) and about the path of the beam 7) is a first accelerating electrede 74 which may have a dish-shaped surface 76 containing an aperture 78. The first accelerating electrode '74- is coupled to the source of potential 68 so as to be maintained at the potential Vm of FIG. 2 substantially negative relative to zero reference potential of the ionizer 50. The ion stream 56 emanating from the emitting surface 52 is thus accelerated from the emitter, past the focusing electrode `titl, and through the aperture 7S of the rst accelerating electrode.

A decelerating electrode 80, of a neutralizing chamber 81., is `disposed forward of the rst accelerating electrode 74 and arranged symmetrically about the path of the ion beam '70. The electrode 80 has a disc portion 82 extending transversely from the axis 54 and a cylinder portion 85 extending forward of the disc portion 82 parallel to the axis 54 of the beam 70. The decelerating electrode S0 is maintained `at a direct current potential such as VD that may be substantially less negative than the potential of the first accelerating electrode '74 as shown by the connection to the source of potential 68 so as to decrease the axial velocity of the ions of the beam '70. In order to develop the potential levell such as 26 of the potential well 30 shown in FIG. 2, a profile shaping electrode 88, which may structurally be considered to be a part of the decelerating electrode 22 of FIG. 1, is provided as cylinder disposed symmetrically about the path of the beam '70. A portion of the electrode 38 adjacent to the decelerating electrode $0 may be disposed within the decelerating electrode This arrangement has been found to provide satisfactory operation but it not considered to be necessarily the only possible arrangement. So that the potential well 3G of the curve of FIG. 2 will have sharp edges, the diameter of the profile shaping electrode 88 is chosen to be less than that of the first accelerating electrode 86 and a second accelerating electrode 110. The profile shaping electrode 88 is essentially a drift tube and extends axially forward a suitable distance to develop the flat potential as indicated by the line 26 of the potential well 30 thus providing a region essentially free of axial electrice fields so that the mean potential of the ion beam 70 is maintained substantially constant therein, except for the self-regulating action, as will be explained subsequently. By a connection to the source of potential 68, as shown, the direct current potential of the profile shaping electrode 88 is maintained at the potential VDI less negative than that of the decelerating electrode 80.

For injecting neutralizing particles, considered here for illustrative purposes to 'be electrons, into the potential well 30, an opening 92 is provided around the profile shaping electrode S8 transverse to the axis 54 which may divide the annular cylinder into two portions. The axial location of the opening 92 is selected so that the electrons are injected into the potential well 30u It is to be noted that the potential maintained at profile shaping electrode 88 is variable so that the 'depth of the potential well is adjustable to control the operation thereof. An electron emitter 98 which, for example, may be a thermionic emitter ring is disposed adjacent to the slot 92. The emitter 98 which may be of the impregnated tungsten type is maintained from the source of potential 68 at a suitable direct current potential such as VD that may be somewhat more negative than the profile shaping electrode 88.

An electron directing electrode 102 including a disc portion 104 and a transverse cylinder portion 106 for developing fields to direct the electrons through the opening 92 is disposed as shown about the profile shaping electrode 88. The directing electrode 102 may have any suitable shape for creating appropriate electron directing fields and may be maintained by the source of potential 68 at a` suitable potential slightly more negative than that of the emitter 98.

When it is desired that the transverse electric field developed by the mean potential of the beam '70 provide control over the electrons emitted from the emitter 98, the prole shaping electrode 88 is of a material and construction so that the profile shaping electrode 88 does not isolate electrons at the emitter 98 from the effects of the potential in the beam, so that the lines of electrical flux developed by the mean beam potential effectively pass through the profile shaping electrode and the opening 92 as through the grid of a low mu vacuum tube. It is to be noted that when the emitter 98 is arranged, as shown, the fiel-d created by the `mean potential of the beam 70 provides a negative feedback control over the number of electrons injected from emitter 98 for one form of the self-regulating action of the scheme in accordance with this invention.

Disposed symmetrically about the path of the beam 70 is the second accelerating electrode 110 which is a cylindrical envelope or housing having a diameter or spacing across the beam larger than that of the profile shaping electrode 88. The second accelerating electrode 110 may have an end 112 adjacent to the profile shaping electrode 88 overl-apping the end of the electrode 88 in the axial direction. The second accelerating electrode 110 is maintained at a suitable direct current potential VA2 by the source of potential 68, which potential is slightly more negative than that of the profile shaping electrode 88 so as to provide the forward potential Wall 27 of the potential valley 30 of FIG. 2. The drift tube region within electrode 110 permits additional interaction between the oppositely char-ged particles of the mixed beam. The beam 70 after being thus neutralized is projected into Kspace through suitable exit structure (not shown) for providing a thrust to the craft.

In operation, which will be explained in reference to the general arrangement of FIG. 1 and to the more specie arrangement of FIG. 3, the charged particles or positively charged particles such as ions, are projected axially as the beam 12 or the stream S6 in a forward direction from the source of FIG. l which may include the ion source #t6 and focusing electrode 60 of FIG. 3, in response to the potential field developed by the rst accelerating electrode 18 which may have the configuration of the rst accelerating electrode 74. The axial electric field at the first accelerating electrode 74 is thus increased to the relatively large negative potential VAI and the ions in the beam 70 have relatively high axial velocities. The geometry of the ion gun or accelerator region including the ion emitter 50, focusing electrode 60 and first accelerator electrode 74 may be selected to provide a convergent flow of the stream 56 for several reasons. Because the lens effect of the aperture 78 in the first accelerating electrode 74 is divergent, a convergent flow in the gun provides a converging beam emerging from the gun into the neutralizing chamber 81. Also, the convergent flow of the stream 56 providing compression transverse to the axial ow saves power required to heat the ionizer 50 by reducing the length of ionizer required to produce aV given ionizer area, thereby reducing edge heat losses. The ion gun also should be designed to minimize interception of ions by the first accelerating electrode 74 (c g. by the method taught in referenced Patent No. 2,817,033).

In relation to the potential of the electrodes, the potential of the first accelerating electrode 74 relative to the potential of the ionizer 50 and the design of the accelerator system is selected as to preclude arcing or voltage breakdown.

The accel.decel. ratio VAl/VD may be selected as a result of a number of considerations. For example, the perveance resulting from the first accelerating electrode 74 must be consistent with the compression of the ion stream 56 and the quality of the optics desired. Also, if the accel.-decel. ratio is too high, the fields in the neutralization chamber 81 will be relatively high for a long axial distance forward of the first accelerating electrode 74, adversely affecting the neutralization operation. The potential of the first accelerating electrode 74 should have a high enough negative potential to ensure that the ion emission density is sufficient to avoid loss of eflciency. Further, it is desirable that the deceleration of the positively charged particles or ions at the decelerating electrode suciently reduces the velocity of the ions to provide the optimum or desired velocity of ejection of the particles. For example, it has been determined that an accel.-decel. ratio VAl/ VD between 2 and 3 provides good overall performance although the invention is not limited to this range.

Before further discussing the operation of the potential well 30, the general requirements of neutralizing a beam of charged particles by injecting neutralizing charges therein will be discussed further. In order for a neutralizing system to provide charge neutrality, the total ion current Ii must be equal to the total electron current Ie averaged over a reasonable period of time. Another requirement of a neutralization system is that the neutralizing charges be ejected from the system with the beam in a manner to prevent the creation of local image charges around the exit region 0f the beam 70 from the second accelerating electrode 110. In an ion engine, electric flux lines existing between the ions and the image charges on the craft will decelerate the ions to develop a counter thrust to reduce the available thrust from the engine. In order that no electric flux lines emanate from the ion beam at the exit of the second accelerating electrode 110, the net charge per unit length in the beam 70 must be zero; that is, qi=qe where q denotes the ion or electron charge per unit length of the ion beam. Considering the requirement for effective neutralization that the total current be zero; for the net charge per unit length to be zero, the axial components of velocity vz, of the ions the axial component of velocity vze of the electrons must be equal.

In the operation of the invention, the electron current is maintained equal to the ion current by adjustment of the potential VDI of the profile shaping electrode 88 so that the -well 30 of FIG. 2 has a desired average potential level 26. The axial velocity of the electrons leaving the well 30 into the beam 70 is controlled by the automatic self-regulation operation of the potential well 30. The axial velocity components of the thermal electrons injected into the beam 70 by the electron emitter 98 possess the Maxwell-Boltzmann distribution of FIG. 4 shown by a curve having a positive and negative axial velocity +V and -V as electrons oscillate between the walls of the Well 30. The positive velocity -i-v of the electrons will be assumed to be in the forward direction, to the right in the figures, of the ion beam 70.

In order that the electrons have a desired axial velocity distribution relative to the axial velocity distribution of the ions, the kinetic energy of the electrons must typically be much smaller than a volt. For example, the electrons must have .03 electron volt of energy when the kinetic energy of the cesium ions is of the order of S kilovolts. However, in the self-regulating system, in accordance with this invention, the electrons may be emitted into the well 3% with l volt kinetic energy as determined by the direct current potential of the electron emitter 98. This kinetic energy, however, is not suicient necessarily to permit the electron to escape the trap represented by the well 3S. Only a predetermined portion of the higher energy electrons will escape, the actual number being determined by the instantaneous level of line 26 which level is in turn controlled by the number of electrons injected.

ln the selregulation operation, the accumulated electrons, as stated above, atleet and control the mean potential of the beam itl in the well 3d which is indicated by the line 26 of FIG. 2 so that electrons i aving a velocity in the well of vH, where vH represents the velocity necessary to overcome the instantaneous height of the wall 27, escape from the well 36 with the desired velocity. As excess electrons accumulate in the well 3Q, the mean beam potential level 26 rises in a negative direction to a level 124, that is, from VD toward Vm and increases the escape rate of the electrons which drives the level 124i down toward Vm. Similarly, when the electron charge decreases in the well 3d, as from an excessive escape rate, the average potential level or mean beam potential 26 falls to a level 126` toward the potential Vm.

To understand the regulatory action of the well 30, assume that the mean beam potential is trst at the level 12,5 resulting from a relatively small number of electrons trapped in the well 30u Assume further that the rate of electron injection from the source 34, or the electron emitter 98 is constant, that is, the source 34 is such that the mean beam potential does not atleet the number of electrons emitted. As electrons accumulate in the well 3d, the mean beam potential 125.` rises to the level 124 allowing electrons with less and less potential energy and axial velocity to overcome the potential barrier of the wall 27 and escape from the well 3d. Thus, a large number of the electrons move forward in the beam itl with an average value of velocity which is selected to be substantially equal to the velocity of the ions. This selection is made by varying the potential of the profile shaping electrode. When the mean beam potential is at the level 124, the escape of electrons from the trap causes the mean beam potential to fall in a positive direction to an equilibrium level. The mean beam potential may oscillate as a result of the selfregulating action of the well 30 so that electrons escape from the well 30 with a desired axial velocity distribution as shown by a curve 132 of FIG. 5 where a line 133 represents the substantially constant axial velocity distribution of the ions. It is to be noted that the mean beam potential level 12.4` to which the well Sti rises may be slightly more positive than the potential VA2; however, it has been determined hat substantially all electrons escape from the well 3d after an appropriate energy exchange between the electrons and the ion or space charge field.

ln the above description the operation of the potential well 3? has been explained when the source of neutralizing particles 34 is not controlled by the mean beam potential. The operation will now be explained when the mean beam potential provides a feedback to control the emission of the neutralizing particles. The arrangement of the electron emitter 98 and profile shaping electrode 88 ot FIG. 3 is an example of an arrangement where the selfregulation action causes the potential level of the well 30 not only to vary or oscillate in response to shortage and accumulation of electrons, but also to control the rate of electron emission. Assuming that the mean beam potential of the well 3% is first at the level 126, the iield developed by the mean beam potential effects the number of electrons emitted from the electron emitter 98 so that the relatively positive mean beam potential attracts an increased number of electrons from the emitter 9S relative to the number when the mean beam potential is VD or Vm. However, the majority of the accumulated electrons have insuiiicient potential energy to overcome the potential barrier of the wall 27 from the average potential level Ve. As electrons accumulate in the well 30, the mean beam potential 126 rises in the negative direction to the mean beam potential 124 so that the accumulated electrons have a decreasing potential barrier to overcome in order to escape from the well 36 and electrons with less and less potential energy escape with a substantially constant axial velocity equal to the desired axial velocity. in effect the velocity vH of FIG. 4 at which electrons have sufficient energy to escape with the ion beam '79 moves to the left so that electrons with decreasing axial velocity leave the well 3d'.

As the mean beam potential 12d increases in negative potential to the level 124 which is more negative than the potential VD of the electron emitter 98, the rate of electron injection is greatly retarded by the field in a i tive feedback or regenerative manner so that a der Vieucy of electrons causes the mean beam potential to fall in a positive direction toward the level 126.

When the mean beam potential falls below the level 26, electron emission from the emitter 9S is again increased due to the negative feedback in a manner so that electrons again accumulate at a fast rate to cause the mean beam potential to rise in a negative direction through the potential level VD to the level 124 to again allow electrons of varying potential` energy to escape from the well 3d. rthus, the depth of the well 30 may iiuctuate as indicated by an arrow 12S, in PEG. 2, and groups of electrons may escape with the beam 7d, primarily during the period when the potential level 26 rises toward the level 124. The fluctuations of the potential level of the well may occur at a rapid rate and are considered to be an aid in effectively neutralizinU the beam '70 forward of the well 30'.

lt is to be noted that in addition to the axial potential trough or well 3%, a trough also exists in the transverse plane or" the beam 7d because the potential distribution across the initially un-neutralized beam has a positive peak at the beam center. This transverse potential distribution further aids in effective neutralization by distributing the electrons transversely throughout the beam 7d.

As previously indicated, the average level 26 of the well 3d is adjusted initially by varying the potential of the profile shaping electrode 88 so that electrons escaping from the Well 36 have a desired velocity distribution. The electron velocity distribution curve 132 of FIG. 5 shows the electron velocity distribution matched closely to the ion velocity distribution indicated as substantially constant by the line 133. Thus by varying the potential of electrode S8, the curve E32 may be shifted along the velocity axis of the graph oi FIG. 5 until the peak of curve 132 coincides with the position of curve 133. In this manner substantially complete time average neutralization is obtained as the self-regulation operation of the invention serves to maintain eifectively this coincidence or the two curves.

The neutralized beam 76 is projected forward through the second accelerating electrode 2S of FIG. 1 which may have the form of the second accelerating electrode 11d of FIG. 3 which electrode forward olf the portion 112 is substantially free of axial fields. The neutralized beam iti is` then projected into space providing an ion engine lwith a relatively high thrust. It is to be noted that the neutralizing scheme in accord-ance with this invention is not to -be limited to an ion engine, but is equally applicable to any neutralization requirement. Also, it is to be noted that the principles, in accordance with this invention, are not limited to neutralization of a beam of positively charged particles lby negatively charged particles as shown in FIG. l Ibut is applicable to neutralization of a bea-m of charged particles of either polarity by particles of opposite polarity injected into the potential Well. Although the source of negatively charged particles 34 of FIG. l `when controlled by the mean beam potential has been described as an electron emitter such as 98 disposed radjacent to the profile shaping electrode 8S so as to be directly responsive to the mean beam potential, other electron sources may be utilized. For example, an electron gun per se may be utilized to respond, in a negative feedback manner as discussed above, to a sensing 'device disposed adjacently to the beam '76. Also, the system, in accordance with this invention, does not necessarily provide control over the electron sources, and, in such cases, a different type of electron 'gun may be utilized.

It is emphasized that the use of the terms electrons and ions and negatively charged particles and positively charged particles in this specification is not intended as a limitation of the invention. In most instances of their use, the terms may be, with the scope of the invention, respectively interchanged by one skilled in the art to indicate, for example, that the neutralizing particles .may be positive and t-he heavier particles negative. Further, it is noted that in `accordance with the principles of this invention, the neutralizing particles may be neutral particles which become ionized upon injection, for example, as by collision, or irradiation.

Thus, there has been described a neutralizing scheme that provides a potential well in a beam of positive particles, for example, and injects negative particles, for example, into the well to be trapped therein. The systems may be self-regulating in that the mean beam potential which is the potential level of the well varies as the accumulated electron charge to allow the negative particles to escape with the beam with a desirable axial velocity distribution substantially matched to the axial velocity distribution of the yaxial velocity distribution of the positively charged particles. Also, the mean beam potential provides a field that may be utilized to control the emission of electrons in a regenerative `fashion so that electrons periodically escape from the well to provide an even narrower velocity distribution. The system, in accordance with this invention has wide use where effective neutralization chambers are required such as in an ion engine.

What is claimed is:

1. A device for neutralizing the space charge of a beam of charged particles projected in a forward direction comprising a first accelerating electrode disposed adjacent to said beam and along the axis of said beam, a decelerat- -ing electrode disposed adjacent to said beam and spaced in the lforward direction from said first accelerating electrode, means for injecting neutralizing particles disposed adjacent to said beam and spaced in a forward direction from said decelerating electrode, and a second accelerating electrode disposed `adjacent to said beam and spaced in a forward direction from said means for injecting neutralizing particles.

2. A device for neutralizing a bea-rn of positively charged particles projected in 4a forward direction comprising a first accelerating electrode disposed adjacent to said beam and along the axis or plane of symmetry of said beam, a decelerating electrode disposed adjacent to said beam Iand spaced in the forward direction from said first accelerating electrode, means for injecting negatively charged` particles disposed adjacent to said beam and spaced in a forward direction from said decelerating electrode, and a second accelerating electrode disposed adjacent to said beam and spaced in a forward direction from said means for injecting negatively charged particles.

3. A device lfor effectively neutralizing a stream of char-ged particles of a first polarity projected in Ia Iforward direction along a predetermined path comprising a first accelerating electrode disposed adjacent to the stream, a first decelerating electrode disposed adjacent to said stream and spaced forward from said first accelerating electrode, electrode means disposed yadjacent to said stream and spaced forward from said decelerating electrode for providing an electric field transverse to said stream, injecti-on means disposed adjacent to said stream and disposed substantially at said electrode means for injecting charged neutralizing particles of a second polarity into said stream, and a second accelerating electrode disposed adjacent to said stream and spaced forward from said electrode means.

4. A device for effectively neutralizing a stream of positively charged particles projected in a forward direction along a pre-determined path comprising a first accelerating electrode disposed adjacent to the stream, a first decelerating electrode Idisposed adjacent to said stream and spaced :forward from said first accelerating electrode, electrode means disposed adjacent to said stream and spaced forward from said decelerating electrode for developing an electric field transverse to said stream, injection means disposed adjacent to said stream and spaced substantially at said electrode means for injecting negatively charged particles into said stream, and a second accelerating electrode disposed adjacent to said stream and spaced rforward from said electrode means.

5. Means for eiecting neutralization of a beam of charged particles moving in a forward direction cornprising a first accelerating electrode disposed adjacent to said beam, a second accelerating electrode disposed adjacent to said beam, a -decelerating electrode disposed adjacent to said beam and between said first and second accelerating elec-trades for developing .a potential well, and a source of neutralizing particles disposed adjacent to said beam for injecting said neutralizing particles into said potential well.

6, A chamber for neutralizing a stream of charged particles moving in a forward direction comprising Ia first accelerating elect-rode mounted adjacent to said stream, la decelerating electrode mounted adjacent to said stream and forward of said first accelerating electrode, a profile shaping electrode mounted adjacent to said stream `and forward of said decelerating electrode for forming a potential well Ito trap neutralizing particles and for maintaining a length of said beam in the valley of said potential Well at a substantial-ly constant potential, a Isec-ond accelerating electrode moun-te-d adjacent to said stream and forward o-f said p-rofile shaping electrode for developing a potential barrier to charged particles of charge polarity opposite to that of :said stream of charged particles moving forward with said stream, and a source of neutralizing particles mounted adjacent to said stream and at said profile shaping electrode for injecting said neutralizing particles into said potential well and for being controlled by the potential of said stream in said well,

7. An ion engine comprising an ion source for emitting v positively charged ions in a forward direction, a focusing electrode -disposed forward of said ion source for focusing said ions into a beam, -a first accelerating electrode disposed adjacent to said ion beam Iand forward of said focusing electrode, a decelerating electrode mounted -adjacent to said beam and forward of said first accelerating electrode, ya profile shaping electrode for forming a potential well -to trap electrons mounted adjacent to said beam and forward of said decelerating electrode, a second accelerating electrode mounted Iadjacent to said beam and forward of said profile shaping electrode, and an electron emitter mounted adjacent to said beam and between said decelerating electrode and said secon-d accelerating electrode for injecting electrons into the potential well.

8. A self-regulating chamber for neutralization of a beam of positively charged particles moving in a forward direction comprising first, second, third, four-th, and fifth sources of potential having successively more negative potential values, a source of the beam of positively charged particles coupled to said first source of potential, a first 'accelerating electrode mounted adjacent to said beam and forward of -said source land coupled to said fifth -source of potential, a decelerating eiectrode mounted adjacent to said beam and forward of said first accelerating electrode an-d coupled to said third source of potential,

a profile shaping electrode disposed forward of said decelerating electrode and coupled to said second source of potential, a source of negatively charged particles disposed adjacent to said profile shaping electrode and coupled to said 4third source of potential for injecting negatively charged particles into said beam, and a second accelera-ting electr-ode disposed adjacent to said beam forward of said profile shaping electrode and coupled to said fourth source of potential, said profile shaping electrode forming a potential well to trap said negatively charged particles so that only negatively charged particles having a predetermined axial velocity range move forward with said beam out of the trap.

9. An ion engine for providing a neutralized beam of positively charged ions comprising first, second, third, fourth and fifth sources of potential having successively relatively more negative values, an ion emitter for emitting ions in a forward direction coupled to said first source of potential, a focusing electrode disposed adjacent to said beam land forward of said ion emitter and coupled to said first source ofy potential, 1a rst accelerating electrode mounted adjacent to said beam and forward of said focusing electrode and coupled to said fifth source of potential, a decelerating electrode mounted adjacent to said beam and forward of said first accelerating electrode and coupled to said third source of potential, 'a profile shaping electrode disposed forward of said decelerating electrode and coupled to said second source of potential, a source of electrons disposed adjacent to said profi-le shaping electrode and coupled to -said third source of potential for injecting electrons into said beam, Iand 'a second accelerat- -ing electrode disposed adjacent to said beam forward of said profile shaping electrode Iand coupled to said fourth source of potential, said profile shaping electrode forming la potential well to trap -said electrons so that only electrons having a predetermined axial velocity range move forward with said beam out of the trap to develop the neutralized beam.

liti. A self-regulating chamber for neutralization of 'a beam of positively charged particles moving in a `forward direction comprising first, second, third, fourth, and fifth sources of potential having successively relatively more negative potential values, a source of the beam of positively charged particles coupled to said 'first sour-ce of potential, la first accelerating electrode mounted adjacent to said beam and forward of said source and coupled to said fifth source of potential, a decelerating electrode mounted adjacent to said beam and forward of said first :accelerating electrode and coupled t-o said third source of potential, a profile shaping electrode disposed forward of said decelerating electrode and coupled to lsaid second source of potential, said pro-file shaping electrode having an opening therein, .a source of negatively charged particles disposed adjacent to the opening of said profile shaping electrode and couple-d to said third source of potenti-al for injecting negatively charge-d particles into said beam, said source of negatively charged particles being controlled by the potential lof `said beam at said profile shaping electrode, and a second accelerating electrode disposed adjacent to said beam forward of sai-d profile shaping electrode and coupled to said fourth source of potential, said profile shaping electrode forming `a potential Well to trap said negatively charged particles so that only negatively charged particles having a predetermined axial velocity range move forward with said beam out `of the trap.

lill. An ion engine for providing a neutralized solid or hollow cylindrical beam of positive-ly charged ions comprising first, second, third, fourth and fifth sources of potential having successively relatively more negative values, an ion emitter having a concave emitting surface for emitting ions in a forward direction coupled to said first source of potential, an annular focusing electrode disposed adjacent to and concentric with said beam and forward of said ion emitter and coupled to said first source of potential, a first annular accelerating electrode mounted adjacent to and concentric with said beam and forward of said focusing electrode and coupled to said fth source of potential, an annular decelerating electrode mounted adjacent to and concentric with said beam and forward of said first accelerating electrode and coupled to said third source of potential, said decelerating electrode having a first internal diameter, .a cylindrical profile shaping electrode disposed adjacent to and concentric with said beam forward of said decelerating electrode and coupled to said second source of potential, said profile shaping electrode having a second internal diameter less than said first internal diameter, said profile shaping electrode having an opening transverse to the beam axis, a thermionic electron emitter ring disposed adjacent to the opening of said profile shaping electrode and coupled to said third source of potential for injecting electrons into said beam, and a second annular accelerating electrode disposed adjacent to and concentric with said beam forward of saidA profile shaping electrode and coupled to said fourth source of potential, said second accelerating electrode having a third internal diameter greater than said second internal diameter, said profile shaping electrode forming a potential well to trap said electrons so that only electrons having a predetermined axial velocity range move forward with said beam out of the trap.

12. Apparatus for neutralizing the charge of a beam of charged particles comprising:

means for producing a field free region through which said beam passes;

means for introducing neutralizing particles into said beam in said region;

electrode means for establishing a first potential barrier to said neutralizing particles at the upstream end of said region;

electrode means for establishing a second potential barrier to said neutralizing particles at the downstream end of said region; and

means for controlling the introduction of said neutralizing particles into said beam in said region such that the beam leaving said region is neutralized.

13. Apparatus for neutralizing the charge of a beam of charged particles comprising:

means for producing a field free region through which said beam passes;

means for providing a source of neutralizing particles adjacent to said beam at said region, said neutralizing particles being charged to a polarity opposite to the polarity of said charged particles;

means for maintaining said source at a predetermined potential;

electrode means for establishing a first potential barrier to said neutralizing particles at the upstream end of said region;

electrode means for establishing a second potential barrier to said neutralizing particles at the downstream end of said region, whereby said potential barriers and said field free region provide a potential well to trap said neutralizing particles; and

means for establishing the mean potential of the charged particles in said beam in said region, with respect to the potential of said second potential barrier,'such that the introduction of said neutralizing particles into said beam in said region is self-regulating whereby the beam leaving said region is neutralized.

14. Apparatus comprising:

means for producing a beam of charged particles;

means for producing a field free region through which said beam passes;

means for introducing differently charged particles into said beam in said region; and

means for establishing a potential barrier to said differently charged particles at the downstream end of said region to trap said differently charged particles such that only differently charged particles having a predetermined axial velocity range move forward with said beam out of said region.

l5. An ion engine for providing a neutralized beam of ions comprising:

means for producing a collimated beam of positively charged ions;

means for producing a field free region through which said beam passes;

means for introducing electrons into said beam as it passes through said region at a rate such that the total electrical charge of the beam is substantially zero as it emerges from said region;

means for providing said region with a first potential barrier to electrons at the upstream end of said region; and

means for providing said region with a second potential barrier to electrons at the downstream end of said region.

16. The apparatus according to claim 15 in which:

said means for producing a field free region includes a hollow, cylindrical, prole shaping electrode maintained ata predetermined potential and having a predetermined length through which said beam passes whereby said region extends a predetermined distance and is essentially free of axial electric fields and whereby the mean potential of the particles in said beam in said region is substantially constant except for the effect of said introduced electrons, and in which said means for introducing electrons comprises an electron source maintained at a predetermined potential, said predetermined potentials being such as to produce a self-regulating neutralization of the charge of said beam.

References Cited UNITED STATES PATENTS 2,809,314 10/1957 Herb 313-63 2,880,337 4/1959 Langmuir 313-63 3,050,652 8/1962 Baldwin 313-63 20 JAMES w. LAWRENCE, Primary Exrmrfner.

C. CAMPBELL, Assistant Examiner. 

